"What are these men doing?"

Translation: Co robią ci mężczyźni?

[Jas789]

Why isn't it tych? Isn't robi Accusative?

[richard.birks]

Who is doing the "doing" ... it's the men, which means they're the subject of your sentence, so "these men" are in the nominative.

Does it become more clear if I write it like this: "These men are doing what?"

[JaniceSchl1]

Why isn't it 'Co ci mężczyźni robią'?

[br0d4]

This word order is also fine, should be accepted.

[AndrewMill284601]

It accepted mine. Is there a difference in emphasis depending on the word order?

[Jellei]

The rule of thumb is that the emphasis and the 'new, most important information' goes at the end of the sentence. Sometimes also if something is put in an unusual position in the sentence, but then of course you need to know what is usual and what's not.

[chb0lingo]

Me : Co robią tymi mężczyźni
Them: Co robią ci mężczyźni

Explanation?

[fiorass]

You got the noun in the right case (Plural nominative) but the pronoun in the wrong one. The declination goes like this:

Singular:

Nom.: ten mężczyzna,

Gen.: tego mężczyzny,

Dat.: temu mężczyźnie,

Acc.: tego mężczyznę,

Inst.: tym mężczyzną,

Loc.: tym mężczyźnie,

Plural:

Nom.: ci mężczyźni,

Gen.: tych mężczyzn,

Dat.: tym mężczyznom,

Acc.: tych mężczyzn,

Inst.: tymi mężczyznami,

Loc.: tych mężczyznach,

[br0d4]

A pronoun must agree with the noun it describes by its number and gender. See also here: Polish pronouns .

[Selina6829254]

How do I know, when I should use 'Czego' and when 'Co'.

[Jellei]

Well, firstly you need to recognize what cases they are.

"co" is both Nominative (Co to jest? = What is it?) and Accusative (Co robisz? = What are you doing?). "czego" is Genitive (Czego potrzebujesz? = What do you need?).

To figure out what case is needed by the question, it may be helpful to try to answer it. It's easier to see what case is used in the answer.